The present invention relates to a device for separating hydrogen in gaseous mixtures containing it or for purifying gaseous hydrogen.
The invention is used particularly within the electronics industry to obtain ultra pure hydrogen containing, for example, less than one part per million of parts (1 ppm) of other gases. It can be used in meteorology where sounding balloons are used which are swollen with hydrogen, in separating hydrogen produced by the reforming of methanol with steam or by electrolysis, in gas preparation with samples for analysis, in isotopic separations or in refining vegetable oils.
The prior art can be illustrated by the patents U.S. Pat. Nos. 2,961,062, 3,368,329, 3,509,694 and JP-A-61 077 602 (World Patent index AN 86-141 337 Derwent Publication Ltd--London, GB).
For low flowrates, usually 0.1 to 100 (Nm.sup.3 /h) normal cubic meters per hour, the purifying devices usually encountered industrially employ the principle of hydrogen permeating through a membrane of a palladium based alloy which is selectively permeable to hydrogen and which acts like a filter, allowing pure hydrogen to pass through it, but preventing gas impurities or other constituents of the gaseous mixture to pass through it.
During practical application of this process, the filter is usually arranged in the form of a tube which is open at one end, this tube or preferably a series of these tubes, being assembled in an outer casing made of stainless steel, or other material, which is inert to the action of the gas or of the gaseous mixture, so as to form a diffusion cell, the impure hydrogen or the gaseous mixture passing under pressure into the casing where it comes into contact with the outer surface of the tube or tubes, the pure hydrogen diffusing through the wall(s) to the inside of the tube, and then passing through the open end or the open ends of the tube or tubes to be collected outside the unit.
The tubes which are usually 50 to 200 micrometers in thickness, for example, are preferably assembled in parallel relationship on a punctured metal disc (or base) made of stainless steel or nickel, for example.
The assembly is realised by way of a brazing containing palladium and silver in such a way that substantially perfect sealing is obtained between the tubes and the punctured base. Usually, the alloy of the brazing has a very high silver content.
The temperature and pressure at which these tubes are used are respectively at least 200.degree. C., for example 200.degree. to 700.degree. C. and 2 to 50 bar (1 bar=0.1 MPa). The combined effect of the temperature and the pressure difference existing between the inside and outside of the tubes can cause the tubes to become distorted and flattened which can be reduced by incorporating within the tubes at least one stiffening element such as a helicoidal spring which maintains the lateral rigidity of the tubes, whilst allowing the gas to pass through them (this technique is described, for example, in French Patent FR-B-1352751).
It is a known fact that the palladium based alloy which contains silver expands by quite a significant value (about 3% linear), irrespective of its composition in the presence of hydrogen. Moreover, only the palladium-silver alloys containing 74 to 79% silver are not subject to a phase change (.alpha. into .beta. or .beta. into .alpha.) in the presence of hydrogen, such a change being accompanied by fritting of the alloy and the appearance of cracks which brings about leakage.
It is noted that the useful life of purifying devices, for example palladium-silver-based ones, which are currently in use commercially is limited by the number of cycles to which they are subjected which comprise frequent operating and stoppage phases. The reasons for this are as follows:
the tubes which may be made of a palladium-silver alloy, and which expand by 3% in hydrogen, are clamped due to their being fixed in the hole of the base where they are located and thus cannot expand freely. They are therefore subject to cold-hammering and significant fatigue;
the alloy of the brazing which is of a very different composition from that of the tube diffuses, during manufacture of the cell, into the tube and thus modifies the composition of the alloy of the tube to a certain extent so that where this happens the desired percentage of silver is no longer present which enables phase changes to be avoided when the hydrogen is being absorbed.
This brings about fritting, a deterioration in the physical properties of the alloy and the start of cracks.